Half a million Americans should consider taking Truvada, a pill that prevents HIV infection. This is the upshot of updated clinical guidelines from the US Centers for Disease Control and Prevention (CDC) and other public heath bodies released last week. Truvada (tenofovir/emtricitabine) has had a rocky start since it was approved by the Food and Drug Administration in 2012. Fewer than 1800 prescriptions were written in the first year it became available, in part because general practice doctors have not been educated about its use, and because it is also a treatment for HIV, so it is associated with having the infection.